drm/amdkfd: use a high priority workqueue for IH work

In systems under heavy load the IH work may experience significant
scheduling delays.

Under load + system workqueue:
    Max Latency: 7.023695 ms
    Avg Latency: 0.263994 ms

Under load + high priority workqueue:
    Max Latency: 1.162568 ms
    Avg Latency: 0.163213 ms

Further work is required to measure the impact of per-cpu settings on IH
performance.

Signed-off-by: Andres Rodriguez <andres.rodriguez@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>